The present invention relates to low dielectric constant nanoporous silica and to improved processes for producing the same on substrates suitable for use in the production of integrated circuits.
As feature sizes in integrated circuits approach 0.25 xcexcm and below, problems with interconnect RC delay, power consumption and signal cross-talk have become increasingly difficult to resolve. It is believed that the integration of low dielectric constant materials for interlevel dielectric (ILD) and intermetal dielectric (IMD) applications will help to solve these problems.
Nanoporous Films
One material with a low dielectric constant is nanoporous silica, which, as a consequence of the introduction of air, that has a dielectric constant of 1, into the material via its nanometer-scale pore structure, can be prepared with relatively low dielectric constants (xe2x80x9ckxe2x80x9d). Nanoporous silica is attractive because it employs similar precursors, including organicsubstituted silanes, e.g., tetramethoxysilane (xe2x80x9cTMOSxe2x80x9d) and/or tetraethoxysilane (xe2x80x9cTEOSxe2x80x9d), as are used for the currently employed spin-on-glasses (xe2x80x9cSOGxe2x80x9d) and chemical vapor disposition (xe2x80x9cCVDxe2x80x9d) silica SiO2. Nanoporous silica is also attractive because it is possible to control the pore size, and hence the density, material strength and dielectric constant of the resulting film material. In addition to a low k, nanoporous silica offers other advantages including: 1) thermal stability to 900xc2x0 C., 2) substantially small pore size, i e at least an order of magnitude smaller in scale than the microelectronic features of the integrated circuit), 3) as noted above, preparation from materials such as silica and TEOS that are widely used in semiconductors, 4) the ability to xe2x80x9ctunexe2x80x9d the dielectric constant of nanoporous silica over a wide range, and 5) deposition of a nanoporous film can be achieved using tools similar to those employed for conventional SOG processing.
Nanoporous silica films have previously been fabricated by a number of methods. For example, nanoporous silica films have been prepared using a mixture of a solvent and a silica precursor, which is deposited on a substrate, eg., a silicon wafer suitable for producing an integrated circuit, by conventional methods, e.g., including spin-coating and dip-coating. The substrate optionally has raised lines on its surface and preferably has electronic elements and/or electrical conduction pathways incorporated on or within its surface. The as-spun film is typically catalyzed with an acid or base catalyst and additional water to cause polymerization/gelation (xe2x80x9cagingxe2x80x9d) and to yield sufficient strength so that the film does not shrink significantly during drying.
The internal pore surfaces of previously prepared nanoporous films are formed of silicon atoms which are terminated in a combination of any or all of the following species; silanol (SiOH), siloxane (SiOSi), alkoxy (SiOR), where R is an organic species such as, but not limited to, a methyl, ethyl, isopropyl, or phenyl groups, or an alkylsilane (SiR), where R is as defined previously. When the internal surface of the nanoporous silica is covered with a large percentage of silanols, the internal surface is hydrophilic and may adsorb significant quantities of atmospheric water. Even if the film is outgassed by heating before subsequent processing, the presence of the polar silanols can contribute negatively to the dielectric constant and dielectric loss. Previously employed methods for overcoming this limitation and rendering the internal pore surfaces of nanoporous silica less hydrophilic include reacting the internal surface silanols with surface modifying agents, including, for example, chlorosilanes or disilazanes. These reactions, which may be conducted in either liquid or gas phases, result in a (SiO)4-xSiRx [wherein x is an integer ranging from 1 to 3] surface which is normally hydrophobic and less polar than the silanol group it replaced.
However, all of the previously employed methods for producing nanoporous silica films used organic functional moieties to provide hydrophobicity. Although these carbon-containing nanoporous silica films (described, for example, in co-owned patent application Ser. No. 09/111,084, filed Jul. 7, 1998) the disclosure of which is incorporated by reference herein in its entirety) exhibit a number of advantages for semiconductor applications, they also have several potential disadvantages including:
1. Oxidation of the carbon content: During semiconductor processing, e.g., during plasma enhanced chemical vapor deposition (PECVD) and etching, following nanoporous silica film deposition, the presence of organic species can lead to problems such as high via resistance (i.e., the prospective integrated circuit is ruined by xe2x80x9cpoisoningxe2x80x9d the interlayer connectors, due to oxidation of the carbon content of organic substituents, resulting in the deposition of undesirable residues from the etching process in the vias). (see, eg., R. J. Hopkins, T. A. Baldwin, S. K. Gupta, May 7-12, 1989, ULSI Symposium, ECS, Allied Signal) which may require additional process steps to rectify.
2. Added mass: For example, the addition of a trimethyl silyl entity (CH3)3Si as a replacement for a hydrophilic surface silanol adds significant mass to the nanoporous silica. All else being constant, the added mass can produce a significantly higher refractive index and dielectric constant which may be undesirable.
3. Strength: Normally, for semiconductor applications, one desires a material with both low dielectric constant and high strength. For nanoporous silica, these two properties must be balanced. For a given dielectric constant (refractive index/density), the density is fixed, at least for a specific chemical composition. With fixed density, the strength of the nanoporous silica is maximized by having the greatest fraction of solid within the skeleton of the film rather than as appended surface groups.
Thus, in view of the need for rapid competitive advances in the art of microprocessor fabrication, there remains a constant need in the art to improve upon previous methods and materials. In particular, there is a need to provide nanoporous silica films with hydrophobic pore surfaces, while minimizing the above described undesirable effects of organic surface moieties. In particular, it is strongly desired to provide such nanoporous silica films with reduced mass at the nano-scale pore surfaces. This later property will provide greater material film strength for a given desired dielectric constant. Thus, for all of these reasons, there remains a need in the art for methods and compositions for producing nanoporous films suitable for the production of integrated circuits that have all of the above-described desirable properties, while minimizing those previously indicated shortcomings of the art.
In order to solve the above mentioned problems and to provide other improvements, the invention provides new methods for effectively producing low dielectric constant nanoporous silica films having a desired range of dielectric constant significantly lower, or having greater strength at the same dielectric constant, than has previously been obtained, while simultaneously avoiding the shortcomings of previously known methods.
Surprisingly, the methods of the present invention are able to achieve this goal by producing nanoporous silica with pore surfaces on which most of the polar silanol (SiOH) functional groups have been replaced by hydrogen functional groups (SiH) and/or a combination of hydrogen functional groups and organic functional groups. The resulting novel pore surfaces also render the produced film somewhat hydrophobic. This is accomplished by employing suitable starting reagents and processes. In particular, the processes of the invention employ SiH and/or SiC (organic) species as surface modification agents, instead of exclusively relying upon surface modification agents based on silicon-hydrocarbon compounds, which have previously been employed. The lower the proportion of organic moieties on the silylated film surface, the lower the mass associated with the pore surfaces, and therefore a correspondingly reduced film density is achieved. This results in an advantageously reduced dielectric constant, while retaining film mechanical strength.
Accordingly, the invention provides novel processes for forming nanoporous dielectric films or coatings on a desired substrate. The novel process includes the steps of
(a) forming a reaction mixture by combining at least one multi-functional alkoxysilane with at least one tetrafunctional alkoxysilane,
(b) recovering the nanoporous film precursor of (a) from said reaction mixture and depositing the same onto a suitable substrate, and
(c) gelling said deposited film to form a nanoporous dielectric coating on said substrate; wherein the multi-functional alkoxysilane is selected from the group consisting of mono-, di- and tri-functional alkoxysilanes.
The multi-functional alkoxysilane is selected from the group having the formula
An-SiHmxe2x80x83xe2x80x83(Formula 1)
wherein each A is independently an alkoxy (Oxe2x80x94R) wherein R is an organic moiety independently selected from the group consisting of an alkyl and an aryl, and wherein n is an integer ranging from 1 to 3; m is an integer ranging from 1 to 3 and the sum of m and n is 4.
A tetrafunctional alkoxylsilane employed in the processes of the invention preferably has a formula of
A4-Sixe2x80x83xe2x80x83(Formula 2)
wherein each A is independently an alkoxy (Oxe2x80x94R) and R is an organic moiety independently selected from the group consisting of an alkyl and an aryl,
In a further aspect of the invention, the alkoxysilane compounds described above may be replaced, in whole or in part, by compounds with acetoxy and/or halogen-based leaving groups. For example, the precursor compound may be an acetoxy (CH3xe2x80x94COxe2x80x94Oxe2x80x94) such as an acetoxy-silane compound and/or a halogenated compound, e.g., a halogenated silane compound and/or combinations thereof. For the halogenated precursors the halogen is, e.g., Cl, Br, I and in certain aspects, will optionally include F.
In yet a further aspect of the invention, the processes of the invention can also include additional optional processing steps to silylate free silanols on nanopore surfaces of the film, with a capping reagent, e.g., trimethylsilyl [TMS, (CH3)3SiO-] or other suitable, art-known hydrophobic reagents, as described, for example, in co-owned U.S. Ser. No. 09/111,084, filed on Jul. 7, 1998, the disclosure of which is incorporated by reference herein in its entirety. This later process is conducted employing surface modification material that includes an effective amount of a surface modification agent. The nanoporous silica film to be treated is present and the film has a pore structure with hydrophilic pore surfaces. The reaction is conducted by contacting the hydrophilic nanoporous silica film with the surface modification material, which is, as previously mentioned, optionally in a liquid or vapor phase. Further, the reaction is conducted for a period of time sufficient for the surface modification agent to penetrate the pore structure of the film and to produce a treated nanoporous silica film having a dielectric constant of about 3 or less. This process also requires that the surface modification agent is hydrogen-containing and that it is suitable for adding hydrogen moieties to the pore surfaces.
The nanoporous silica dielectric film is optionally produced on a desired substrate by the processes of the invention, or by other art-known processes, prior to treatment by the following process steps. Typically, the film has a nano-scale pore structure with hydrophilic pore surfaces.
In yet another optional aspect, the films produced by the methods of the invention are further coated, e.g., by art-standard spin-on-glass silicon-based polymer precursors, including, but not limited to, LOSP(trademark) and/or HOSP(trademark) siloxanes (low and high organic siloxane polymers, respectively) that are commercially available from AlliedSignal Advanced Microelectronic Materials (Sunnyvale, Calif.). The high or low organic content siloxane film is typically used as an etch-stop or a hardmask, similar to standard SiO2, SiON or SiN in a variety of integration techniques including subtractive aluminum, and damascene and dual damascene processes, where appropriate. It has unexpectedly been found that the addition of such an overcoating of art-standard spin-on-glass silicon-based polymer precursors can enhance the hydrophobicity of the nanoporous silica film surface, e.g., particularly when using high or low organic siloxanes, oxygen resistant siloxanes, and similar silicon based polymer precursors. This overcoating also unexpectedly improves the mechanical strength of the treated nanoporous silica film when nearly any suitable spin-on-glass type of silicon-based polymer precursor is employed.
Thus, in one preferred embodiment, the second dielectric composition comprises a polymer having a structure selected from the group consisting of Formulas 3-10:
xe2x80x83[H-SiO1.5]n[R-SiO1.5]m,xe2x80x83xe2x80x83(Formula 3)
[H0.4-1.0SiO1.5-1.8]n[R0.4-1.0SiO1.5-1.8]m,xe2x80x83xe2x80x83(Formula 4)
[H0-1.0-SiO1.5-2.0]n[R-SiO1.5]m,xe2x80x83xe2x80x83(Formula 5)
[H-SiO1.5]x[R-SiO1.5]y[SiO2]z,xe2x80x83xe2x80x83(Formula 6)
wherein the sum of n and m, or the sum or x, y and z is from about 8 to about 5000, and m and y are selected such that carbon containing substituents are present in an amount of less than about 40 Mole percent; and wherein R, is selected from substituted and unsubstituted straight chain and branched alkyl groups, cycloalkyl groups, substituted and unsubstituted aryl groups, and mixtures thereof;
[HSiO1.5]n[RSiO1.5]m,xe2x80x83xe2x80x83(Formula 7)
[H0.4-1.0OSiO1.5-1.8]n[R0.4-1.0SiO1.5-1.8]m,xe2x80x83xe2x80x83(Formula 8)
[H0-1.0SiO1.5-2.0]n[RSiO1.5]m,xe2x80x83xe2x80x83(Formula 9)
wherein the sum of n and m is from about 8 to about 5000 and m is selected such that the carbon containing substituent is present in an amount of from about 40 Mole percent or greater; and
[HSiO1.5]x[RSiO1.5]y[SiO2]z;xe2x80x83xe2x80x83(Formula 10)
wherein the sum of x, y and z is from about 8 to about 5000 and y is selected such that the carbon containing substituent is present in an amount of about 40 Mole % or greater; and wherein R, is selected from substituted and unsubstituted straight chain and branched alkyl groups, cycloalkyl groups, substituted and unsubstituted aryl groups, and mixtures thereof
In another optional aspect of the invention, the films produced by the methods of the invention are further coated, e.g., by art-standard spin-on-coating, with copolymer compositions known to the art as oxygen plasma resistant poly(hydrido siloxane compounds having a general formula of.
(HSiO1.5)a(HSiO(OR))b(SiO2)c,xe2x80x83xe2x80x83(Formula 11)
are provided, wherein R is a mixture of H and an alkyl, group having from 1 to 4 carbon atoms; a+b+c=1; 0.5 less than a less than 0.99; 0.01 less than b less than 0.5; and 0 less than c less than 0.5.